Technical Field of the Invention
The present invention relates to a motor control apparatus for a hybrid vehicle that has an engine and a motor as motive power sources of the vehicle.
Related Art
Recently, hybrid vehicles have come to capture attention. In light of the growing societal needs for good fuel efficiency and low emission of exhaust gas, hybrid vehicles have an engine and a motor installed as motive power sources of the vehicles. As disclosed in JP-A-2009-143360, for example, such a hybrid vehicle includes a system which is provided with a first element (e.g., the rotating part of a dog clutch) whose rotation is controlled by the motor and a second element (e.g., the stationary part of the dog clutch) which is brought into engagement with the first element. The first and second elements are synchronized in their rotation phase by allowing the motor to output predetermined torque. In this system, an inertial torque of the motor shaft is calculated on the basis of the inertial masses of the engine and the motor and the rotation speed of the motor. Then, the predetermined torque is corrected by the so inertial torque before being outputted to the motor.
The inventor of the present invention pursues research on a system for controlling rotation speed of a motor (hereinafter this control is referred to as “motor rotation speed control”) in a hybrid vehicle in which the motor is connected to the power train of engine. In the motor rotation speed control performed in this system, when predetermined conditions are met (e.g., when the clutch located between the motor and the transmission is in a disconnected state), a torque command value is calculated under PI (proportional plus integral) control so that the deviation between a target rotation speed and an actual rotation speed of the motor is reduced. In the process of the research, a new issue as set forth below has been revealed.
As shown in FIG. 1, in a hybrid vehicle in which the motor is connected to the power train of engine, rotation fluctuation (fluctuation in the actual rotation speed) may be caused in the motor, while the motor rotation speed control is performed, being influenced by the rotation fluctuation of the engine, which is induced by combustion. In such a case, when the gain of the PI control is large, the torque command value for the motor may vary (oscillated) due to the fluctuation in the actual rotation speed of the motor, raising a problem of causing instability in the torque of the motor. As a measure against this, the gain of the PI control may be reduced to suppress the variation of the torque command value for the motor, which variation is caused by the rotation fluctuation of the engine. However, reducing the gain of the PI control may raise another problem of impairing the responsiveness of the motor rotation speed control, thereby increasing the rotation speed deviation (deviation of the actual rotation speed with respect to the target rotation speed) of the motor.